Leaks often occur in membrane-based systems due to a variety of defects such as: pinholes, scratches or other defects in membranes; failed glue lines, seals and potting within membrane module construction; and misaligned seals or inter-connectors between modules within a membrane-based system. One known technique for testing systems for such leaks involves the introduction of particles into the feed liquid upstream from the membrane. The particles used in such testing typically have a size larger than the nominal cut-off of the membrane such that the detection of particles within the permeate can be correlated to a leak or defect within the system. Examples of particles used in such testing include: fluorescent microspheres as described in US 2008/0105038, latex particles as described in U.S. Pat. No. 5,480,554 and magnetic particles such as ferrites and transition metal oxides, sulfides, silicides and carbides as described in U.S. Pat. No. 7,011,758.
One aspect of such testing techniques is that the concentration of particles introduced within the feed liquid is much greater than that measured within the permeate solution, (e.g. often 1000 or event 100,000 times greater). At such high concentrations, many particles are cost prohibitive. Moreover, when used in high concentrations many particles foul the membrane, catalyze scale formation, and in some instances, catalyze the oxidation of the membrane. Depending upon the particle type and application, particles may also need to be recovered from the permeate and/or reject solution.